(1) Field of the Invention
This invention relates to a semiconductor device, an electrode member, and an electrode member fabrication method and, more particularly, to a semiconductor device used in a high-current high-voltage operating environment, an electrode member used in such a semiconductor device, and a method for fabricating such an electrode member.
(2) Description of the Related Art
In recent years power modules which can withstand high-current high-voltage operating environments have been utilized in power converters, such as inverters/converters, for driving motors included in robots, machine tools, electric vehicles, and the like. At present, such power modules are made up mainly of power semiconductor elements, such as insulated gate bipolar transistors (IGBTs) and free wheeling diodes (FWDs) (see, for example, Japanese Unexamined Patent Publication No. 2004-6603).
FIG. 17 is a schematic sectional view of an important part of a conventional power module.
FIG. 17 shows how an IGBT 100 is mounted in a power module. Usually an emitter electrode and a gate electrode (hereinafter referred to as the “emitter electrode etc.”) are formed on one side of the IGBT 100 and a collector electrode is formed on the other side of the IGBT 100. In FIG. 17, it is assumed that the IGBT 100 is mounted with the emitter electrode etc. upward and the collector electrode downward. As shown in FIG. 17, for example, the emitter electrode etc. formed on the upper side of the IGBT 100 are connected to corresponding external connection terminals (not shown) by aluminum wires 101.
On the other hand, the collector electrode formed on the under side of the IGBT 100 is located on an insulating board 102 on a copper (Cu) heat radiation base (not shown and herein after referred to as the “copper base”). The insulating board 102 is a ceramic plate made of, for example, alumina which transmits heat well. Copper leaves 102a and 102b are attached to both sides of the insulating board 102. The copper leaf 102a attached to the upper side of the insulating board 102 is soldered onto the collector electrode and the copper leaf 102b attached to the under side of the insulating board 102 is soldered onto the copper base. By adopting such a structure, electrical connection between the IGBT 100 and the outside can be secured, insulation between the IGBT 100 and a heat radiation system can be secured, and heat generated at operating time can be transmitted to the copper base via the insulating board 102.
With the above conventional mounting method, heat can be radiated from the under side of the IGBT 100 via the insulating board 102 and the copper base. However, only the thin aluminum wires 101 with a diameter of, for example, about 300 to 400 μm are connected to the upper side of the IGBT 100. In addition, the aluminum wires 101 generate heat as a result of sending an electric current. Accordingly, it can hardly be hoped that heat will be radiated from the upper side of the IGBT 100. Moreover, heat generated by the aluminum wires 101 may degrade the characteristics of the element. It is possible to use copper wires having high thermal conductivity in place of the aluminum wires 101. However, usually a certain number of wires corresponding to current capacity are ultrasonic-bonded onto the surface of the IGBT 100. Therefore, it is desirable that copper wires the hardness of which is higher than that of the aluminum wires 101 should not be used so as not to damage the surface of the element.
FIG. 18 is a schematic sectional view of an important part of another conventional power module. Components in FIG. 18 that are the same as those shown in FIG. 17 are marked with the same reference numerals and detailed descriptions of them will be omitted.
To avoid the problems which arise by the use of an aluminum wire, an attempt shown in FIG. 18 has conventionally been made. In FIG. 18, a copper electrode 103 is soldered onto the upper side of an IGBT 100 where an emitter electrode etc. are formed. An external connection terminal 104 drawn out of the power module is bonded to the copper electrode 103. By doing so, electrical connection between the IGBT 100 and the outside is secured and heat can also be radiated from the upper side of the IGBT 100 via the copper electrode 103.
The same applies to the case where an FWD is mounted in a power module. For example, an anode electrode is formed on the upper side of the FWD and a cathode electrode is formed on the under side of the FWD. Aluminum wires are ultrasonic-bonded onto or a copper electrode is soldered onto the upper side of the FWD. The under side of the FWD is soldered onto a copper leaf attached to an insulating board on a copper base.
However, the thermal expansivity of silicon (Si) which is the main component of the IGBT or the FWD is about 2.6 ppm/° C. On the other hand, the thermal expansivity of copper is about 17 ppm/° C. and is higher than that of silicon. Therefore, if a copper electrode is soldered onto the upper side of the IGBT or the FWD in the above way in place of aluminum wires with heat radiation taken into consideration, a soldered interface is subject to thermal stress at the time of heat cycling or power cycling because of the difference in thermal expansivity between them. Distortion is caused by this thermal stress. As a result, a crack may appear and the target life of the power module may not be realized.
A crack may also appear under the IGBT or the FWD. That is to say, the thermal expansivity of the insulating board which is made of alumina and to the surface of which the copper leaf is attached is about 7 ppm/° C. This insulating board is soldered onto the copper base the thermal expansivity of which is high. Accordingly, a soldered interface is subject to thermal stress because of the difference in thermal expansivity between them. Distortion is caused by this thermal stress and a crack may appear. It is known that if heat cycling is performed between, for example, −40 and +125° C., a crack begins to appear due to distortion at the soldered interface between the insulating board and the copper base after about 500 cycles.
The reason for using copper as a material for heat radiation bases in power modules is that copper has good thermal conductivity (about 350 W/(m·K)). However, to avoid the appearance of such a crack, a material, such as the one including copper molybdenum (CuMo) or aluminum silicon carbide (AlSiC), the thermal expansivity of which is close to 7 ppm/° C. has been used in place of copper. The thermal expansivity of these materials is lower than that of copper, but their thermal conductivity is low (about 150 W/(m·K)). This characteristic is disadvantageous to recent small-loss IGBTs and FWDS. In addition, the costs of manufacturing heat radiation bases by using these materials are about twenty times higher than the manufacturing costs of copper bases.